Plastic parts have traditionally been marked or decorated by hot stamping, silk screening, or painting. In each of these operations, the marking or decoration is placed on the exterior of the plastic part in a secondary operation. One major disadvantage of these techniques is that the marking or design, being on the exterior surface of the part, is subject to degradation by erosion or wear. For example, plastic keypad buttons on typewriters and computer keyboards are subject to constant wear by human fingers actuating the keys. Over a period of time, the printed ink on the keys becomes worn away, making the keyboard unsuitable for use by an unskilled typist. Chemists have toiled countless hours in attempts to create a more permanent inking composition, with varying degrees of success.
One way of overcoming this problem is to actually mold the desired indicia into the plastic. A technique known as `double shot molding` forms the first part of the object in one color of plastic. The remainder of the part is then molded in a second step or shot in a contrasting color, such that the two contrasting color plastics are exposed on the surface of the part, creating the desired indicia. This solves the problem of wear and erosion, but is costly because each part must have a specifically designed mold, and the two shot molding process consumes more molding time, thus increasing the finished cost of the part. If unique or custom designs are required, this process is not a feasible solution.
Other solutions are to cover the printed graphics by laminating a clear layer of plastic over the indicia. This also requires a secondary operation, and may not be suitable for all applications. It can require that the part be reheated, causing it to warp or distort. Another alternate technique is known as in-mold heat transfer decorating. In this process, two operations are performed simultaneously, thus eliminating the labor, time, and space required for secondary functions. Plastic parts are injection molded and decorated in one shot on the molding machine. The process uses a film or foil of gravure printed graphics. The graphics are preprinted and imbedded behind a layer of clear plastic. The film is registered in the proper position in the mold, the mold closes, and the plastic part is formed by conventional injection molding. During the molding process, the film becomes integral to the finished part, thereby creating the decoration on the part. Although this process solves many of the prior art problems, it still has the disadvantage of not allowing for flexibility in custom decorations or markings. Each design requires a unique label or foil, and many parts must be stocked.
Clearly a need exists in the industry for a process that can be used to fabricate a plastic part having indicia, that provides for rapid and easy customization of the finished marking.